Some form of comment previewing (live and optional would be my preference) is long overdue to be included with the default distribution. It's useful to provide a choice of seeing what marked-up comments look like *before* posting so corrections can be made.

The thing that stinks about having this functionality in the plugin and not built into WordPress is lack of customization. Not many WordPress users have the time and ability to learn how to manipulate code of plugins, so it can be hard to customize plugins to the needs of users.

Hence if this functionality was built into WordPress, the idea would be to have an interface to manipulate live comment previewing via the admin console. For instance, I may want the preview to appear above where the user types in the comment. Whereas someone else may want the preview to appear below.

But all that aside, if your competitors have the functionality built in, so should you.